Zeolites are, as will be known, crystalline alumosilicates having ordered channel and cage structures, whose pore openings are in the micropore region of less than 0.9 nm. The network of such zeolites is composed of SiO.sub.4 and AlO.sub.4 tetrahedra joined together via common oxygen bridges. A survey of the known structures may be found for example in W. M. Meier and D. H. Olson, Atlas of Zeolite Structure Types, 2nd edition, Butterworths, London 1987.
To balance out the negative electrovalence due to the incorporation of Al(III) into the Si(IV) silicate lattice, zeolites are found to contain exchangeable cations, in particular, depending on the method of preparation, cations of sodium, potassium, lithium or cesium. Replacing these cations for protons, for example by ion exchange, gives the corresponding acidic solids with the zeolite structure in the H-form.
U.S. Pat. No. 3,329,481 discloses materials in which the Si(IV) in the silicate lattice is said to be replaced by titanium in the form of Ti(IV). These titanium zeolites, in particular those having a crystal structure of the MFI type (cf. Meier, Olson, loc. cit.), and methods for preparing them are described for example in U.S. Pat. No. 4,410,501, EP-A-311,983, U.S. Pat. No. 4,666,692, DE-A-30 47 798 or BE-A-10 01 038. Titanium-containing zeolites having other structures are known from EP-A-405,978. Apart from silicon and titanium, these materials may also contain additional elements such as aluminum (DE-A-31 41 238), gallium (EP-A-266,825), boron (U.S. Pat. No. 4,666,692) or small amounts of fluorine (EP-A-292,363).
Titaniumzeolites with an MFI structure are known to be identifiable from a certain pattern in their X-ray diffractograms and also from a structural vibration band in the IR region at about 950 cm.sup.-1 (DE-A-30 47 798) and hence to be distinguishable from alkali metal titanates or crystalline and amorphous TiO.sub.2 phases.
Titanium zeolites with an MFI structure are known to be suitable for use as catalysts for oxidation reactions (B. Notari, Stnd. Surf. Sci. Catal., 37 (1987), 413-425). For instance, EP-A-100,118 discloses a process whereby propene can be epoxidated with hydrogen peroxide in aqueous phase to propylene oxide over titanium zeolites. The preparation of cyclohexanone oxime from cyclohexanone by reaction with ammonia and hydrogen peroxide is taught in EP-A-208,311 and the hydroxylation of aromatics is known from GB-A-21 16 974. The oxidation of saturated hydrocarbons from C.sub.2 to C.sub.18 with H.sub.2 O.sub.2 over the abovementioned titanium zeolites is described in EP-A-376,453.
The aforementioned titanium zeolites are typically prepared by reacting an aqueous mixture of an SiO.sub.2 source, a titanium dioxide and a nitrogen-containing organic base, for example tetrapropylammonium hydroxide, in a pressure vessel at elevated temperature over a period of several hours or a few days in the absence or presence of alkali. The crystalline product is filtered off, washed and dried and then calcined at elevated temperature to remove the organic nitrogen base. In the powder thus obtained at least some of the titanium is present within the zeolite structure in varying proportions with four-, five- or six-fold coordination (Behrens et all, J. Chem. Soc., Chem. Commun. 1991, 678-680). To improve the catalytic properties, the titanium zeolite powder may additionally be subjected to a repeated wash with a hydrogen peroxide solution acidified with sulfuric acid, after which the titanium zeolite powder must be dried and calcined again, for example as described in EP-A-267,362. The pulverulent titanium zeolite must then be finally processed in a shaping operation with additions of suitable inert binder to obtain a handleable catalyst. A method for this is described in EP-A-200,260.
Existing methods for preparing titanium zeolites with an MFI structure have some serious disadvantages.
For instance, tetrapropylammoniumhydroxide TPAOH that contains only a small residue of alkali metal, in particular potassium, is a very costly starting material which must be used in high concentrations. The processing of the titanium zeolite powder prepared by means of the TPAOH by shaping to fabricate handleable catalysts is labor intensive and after an extrusion with or without an inert binder requires a further energy and time intensive drying and calcining step.